Modern high capacity, long distance communication systems are usually based on fiber-optical data transmission. However, the signals will become more or less distorted due to various linear and nonlinear phenomena upon transmission over the optical transmission lines. In particular, chromatic dispersion (CD), polarization mode dispersion, chirp, extinction ratio, four wave mixing, self phase modulation and cross phase modulation are relevant for the distortions and thereby introduce intersymbol interference (ISI). Additional distortions may be introduced by various parasitic elements of the conversion circuitry.
Generally, these effects reduce the eye opening at the receiver and thereby lead to a reduced tolerable optical signal to noise ratio (OSNR).
In the optical receiver, the optical pulses are converted back into electrical signals. The digital data and sampling clock has to be derived from the analog signal by means of a clock and data recovery circuitry (CDR).
In order to improve the signal quality at the CDR circuit, it is known to apply adaptive equalization. The eye opening penalty caused by ISI may be reduced or removed by employing adaptive equalization, whereby the signal equalization is usually performed in the electrical domain of the optical receiver.
Additionally, forward error correction is frequently applied in order to increase the transmission performance for a given signal to noise ratio. In encoding for forward error correction, redundant bits are added to an incoming bit stream so that errors in transmission may be detected and corrected at the far end.
Different FEC-Coding-Schemes are used, such as so called in band or out band, BCH (Bose-Chaudhury-Hoequengheen) or RS (Reed-Solomon) codes which fit for Sonet/SDH digital wrapper formats. If the input error rate of the data stream is below the error correction capability of the respective error correction code, the bit errors can be corrected and estimates of a bit error ratio (BER) may be measured by using the additional information from the respective FEC-decoding scheme. Specifically, the number of errors that can be corrected amount to (d−1)/2, where d denotes the minimum number of bit positions by which code words for a particular code are different (“Hamming distance”). Thus, using FEC, the BER of the decoded output signals can be greatly reduced in comparison to the incoming signals received and converted by the optical receiver.
In order to increase the eye-opening before digitizing the received and converted signals, linear and non-linear equalizers are employed. Well known filters are feed forward equalizers (FFE) and decision feedback equalizers (DFE).
Particularly, decision-feedback equalization (DFE) is a widely-used technique for removing intersymbol interference where noise enhancement caused by a linear feed-forward equalizer (FFE) may introduce performance problems.
In order to digitize the received signals, currently implemented DFE structures utilize a regular sampling phase derived from the recovered clock by means of a narrowband clock recovery. Consequently, the derived sampling phase is slowly adapted and used for digitization of a large number of bits.
However, the optimum sampling instant or phase may vary depending on the signal history. Particularly, non-linear distortions may cause the position of the maximum eye opening to shift in time so that a CDR using a regular slowly varying sampling phase will miss the optimum instant for signal sampling.